Epoxy resin based adhesives are used to bond a variety of different substrates together.
In certain applications, the adhesive must maintain good bonding to the substrate and good impact resistance over a very wide temperature range. For example, epoxy resin adhesives are used in the automotive industry in metal-metal bonding in frame and other structures. Adhesive bonding can reduce the number of welds that are needed to construct the frame, and for that reason, the use of these adhesives can reduce assembly costs. The adhesive will be subjected to a very wide range of temperatures during subsequent manufacturing processes and during the lifetime of the vehicle. These temperatures may be as high as 80° C. Automobiles that are used in cold climates may be exposed to temperatures as low as −40° C.
Structural adhesives potentially offer similar advantages in aerospace manufacturing as they do in the automotive sector, such as reduced vehicle weight and reduced manufacturing costs. However, aircraft are routinely exposed to temperatures as low as −60 to −70° C. when they operate at altitudes of 30,000 feet or more, which is common in the industry. Structural adhesives used in these applications must retain adequate adhesion and impact resistance at these temperatures.
Many structural adhesives used in automotive applications are based on a rubber-modified epoxy resin and a reactive “toughener”. Structural adhesives of these types are described in, for example, U.S. Pat. Nos. 5,202,390, 5,278,257, WO 2005/118734, U.S. Published Patent Application No. 2005/0070634, U.S. Published Patent Application No. 2005/0209401, U.S. Published Patent Application 2006/0276601 and EP-A-0 308 664. Unfortunately, these structural adhesives tend to exhibit a substantial drop in performance at temperatures of −40° C. or below. It would be desirable to provide a structural adhesive that has good adhesive and impact properties, and which retains those properties better at temperatures as low as −60 to −70° C.
U.S. Patent Publication No. 2011/0114257 describes an impact modifier containing carboxylic acid group(s), which is prepared from the reaction of an intramolecular anhydride of a di- or tricarboxylic acid with at least one amphiphilic block copolymer containing at least one hydroxyl group. The impact modifier is blended with an epoxy resin and is purported to provide improvements in impact resistance at temperatures above or approaching −40° C.
In WO 2005/007720 and U.S. 2007/0066721, an adhesive system is described which contains a polytetrahydrofuran-based toughener based on polytetrahydrofuran (PTHF, also known as polytetramethylene glycol, PTMEG, polytetramethylene oxide, and PTMO). WO 2005/007720 and U.S. 2007/0066721 describe tougheners based on PTHF polymers having various molecular weights. In those systems, the molecular weight of the PTHF is reported to have little impact on adhesive properties.
However, there still exists a need for adhesive compositions having improved impact resistance at temperatures near and below −40° C.